It is highly desirable for tires to exhibit good traction characteristics on both dry and wet surfaces. However, it has traditionally been very difficult to improve the traction characteristics of a tire without compromising its rolling resistance and tread wear. Low rolling resistance is important because it will enhance fuel economy. Good tread wear is an important consideration because it is generally an important factor that determines the life of the tire.
Overall, tire characteristics, such as traction, tread wear, and rolling resistance, are dependent to a large extent on the dynamic viscoelastic properties of the elastomers utilized in making up a tire. But, these tire properties may also be affected by the fillers and additives that are used in preparing the rubber compositions that make up the tire.
It is known that silica fillers may improve rubber properties, such as increased tear resistance, reduced rolling resistance and increased road traction, and thus improve fuel economy for vehicles. But, one difficulty encountered with increasing the silica content is a reduction in processability for green rubber compositions. A rubber composition containing a high level of silica exhibits high green viscosity values, which can make it very difficult to process or extrude the high silica content rubber at reasonable extrusion rates.
Some approaches to improving the processability, i.e. the green viscosity, of high silica content rubber compositions have focused upon the addition sequence of ingredients during mixing, the addition of de-agglomeration agents and using elevated levels of coupling agents. Another recent approach is to increase the amount of processing aids, such as stearic acid.
Unfortunately, higher levels of processing aids, particularly stearic acid, can have a negative effect because excess stearic acid tends to gradually migrate to the surface of the rubber composition and create a surface bloom thereon. This stearic acid migration tends to reduce the coefficient of friction for the rubber surface.
Accordingly, a challenge exists whereby desirable wet traction characteristics of cured rubber compositions are balanced against the processability of its corresponding green rubber composition. Therefore, what is needed is a rubber composition and a process for making same, wherein the rubber composition maintains a desirable green viscosity value, but possesses enhanced wet tractions characteristics when cured, without sacrificing other desirable characteristics.